Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. In this regard, materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being used for high temperature applications, such as for combustor and other hot section components of gas turbine engines, because of the excellent capacity of these silicon materials to operate at higher temperatures.
However, it has been found that silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200° C. will exhibit weight loss and recession at a rate of approximately 152.4 microns per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x.
Methods, such as described in U.S. Pat. No. 6,410,148, the disclosure of which is hereby incorporated by reference in its entirety, has dealt with the above problem concerning use of the silicon based substrates by providing a sufficient environmental barrier coating (EBC) for silicon containing substrates which inhibits the formation of volatile silicon species, Si(OH)x and SiO, thereby reduce recession and mass loss, and which provides thermal protection to and closely matches the thermal expansion of the silicon based substrate. U.S. Pat. No. 6,410,148 describes using an EBC comprising barium strontium aluminosilicate (BSAS) to protect the silicon based substrate. In further embodiments, an intermediate layer is described for providing adhesion between the substrate and/or to prevent reactions between the BSAS barrier layer and the substrate. Still further a bond layer between the immediate layer and the substrate may also be provided which includes silicon.
Although Barium-strontium-aluminosilicate (BSAS) coatings have been shown to provide excellent environmental protection and good thermal barrier protection to silicon based components exposed to temperatures of up to about 2500° F. (1371° C.), these systems may encounter problems when the EBC and the component are subjected to higher operating temperatures above 2500° F. In particular, for application temperatures approaching the melting temperature of BSAS (about 1700° C.), these BSAS protective coating may require a thermal-insulating top coat. U.S. Pat. No. 5,985,970 to Spitsberg et al., the disclosure of which is hereby incorporated by reference in its entirety, mentions the use of a top coat comprising 7% ytrria stabilized zirconia (7% YSZ) as a top layer to a BSAS bond coat for solving this problem.
Further still, as application temperatures increase further beyond the thermal capability of a Si-containing material (limited by a melting temperature of about 2560° F. (about 1404° C. ) for silicon), conventional TBC's mentioned above may not adequately protect the underlying component. Rather, under elevated temperatures approaching 3000° F. or greater, still thicker coatings capable of withstanding higher thermal gradients may be required. However, as coating thickness increases, strain energy due to the CTE mismatch between individual coating layers and the substrate increases as well, which can cause debonding and spallation of the coating system. In order to combat this problem, U.S. Pat. No. 6,444,335 to Wang, et al., the disclosure of which is hereby incorporated by reference in its entirety, describes adding a CTE transition layer between the EBC, e.g. BSAS and the TBC, YSZ for ensuring adherence of the TBC layer to the EBC.
While, the transition layer, EBC, TBC combination of the '335 patent was an improvement over prior methods for running components at higher operating temperatures between about 2500° F. (1371° C.) to 3000° F. (1649° C.), the TBC/EBC system of the '335 patent when subjected to higher operating temperatures may not provide optimum thermal and/or environmental protection to their underlying silicon based component.
After exposure to temperatures of about 3000° F. (1649° C.) and above, the electron beam physical vapor deposited (EP-PVD) columns of the TBC's (YSZ) of some of the prior systems may become subject to sintering, wherein a pulling in leaving large gaps between the columns results. When the above sintering occurs, the TBC layer may have limited protective capability and provide a direct route of attack to the EBC and/or underlayers of the TBC. For example, cracks may continue into the underlying EBC and sometimes through the BSAS layer when the TBC has been subject to sintering or spallation. Additionally, the thermal conductivity of the sintered top coat layer increases, undesirably changing the thermal insulating properties of the coating system.
Moreover, a reduction of the distinct EB-PVD (electron beam-physical vapor deposition) columnar structure due to sintering of adjacent columns is possible. This column sintering will reduce the strain tolerance of the structure and can result in increased interfacial strain and early spallation of the coating. Additionally at extended times at higher operating temperatures of about 3000° F. (1649° C.) and above, some prior TBC (YSZ) systems have a phase instability, which leads to degradation of the TBC coating and ultimately the TBC/EBC system.
Accordingly, there is a need in the art for an improved TBC for use in an TBC/EBC system which provides sufficient thermal and environmental protection to underlying silicon based substrate components operating at temperatures of about 3000° F. (1649° C.) or higher for short or extended periods of time. In particular, an improved TBC is needed which has improved resistance to sintering and improved phase stability for use with a sufficient EBC for coating a silicon containing material substrate.